This invention relates to a method for processing an input signal to generate an output signal, and to applications of that method in hearing aids and listening devices.
So-called transient limiters are signal processing units which are capable, without any delay or overshoot, of limiting rapidly ramping, short-duration i.e. transient signal components to a predefined level, hereinafter referred to as the threshold value. The threshold value concerned, at which the transient limiter performs its function, is not signal-dependent but can instead be set as a parameter. Transient limiters are employed for instance in hearing aids which serve to compensate for a patient's hearing impairment, but also in auditory amplification systems which are used for enhancing audibility in special situations such as listening and monitoring operations. In the following description the term “hearing aid” is to be understood as comprehensively referring to the medical hearing aids and to the listening devices mentioned above. Where any of the following elucidations relate uniquely to applications in listening devices, they will be explicitly identified as such.
In hearing aids, transient limiters serve the purpose of preventing the maximum output level in the hearing aid from exceeding a predefined threshold value. This protects the user of the hearing aid from excessive noise exposure.
It is a known fact that human speech occupies a dynamic range of about −15 to +18 dB (decibels) around the respective mean level; in quiet surroundings with little ambient noise, that mean level is about 60 to 65 dB. In loud surroundings the mean level can rise to about 80 dB or higher. Given these facts, the conventional methods for limiting the audio signal components for persons with normal hearing have employed fixed maximum values of 100 to 120 dB. In cases of diminished hearing capacity the threshold value is suitably set at a comfortable maximum level below the threshold of pain for the patient or user.
It is a characteristic aspect of human auditory perception that not only sounds above the maximum threshold value or comfort level are bothersome. Indeed, it is also transient sounds (such as intermittent noise), even when at a level distinctly below the maximum threshold value, that are perceived as unpleasant in an otherwise prevalently quiet environment. For example, in quiet surroundings, the transient noise of dishes and cutlery, even if well below the maximum threshold value of 100 to 120 dB, creates an unpleasant auditory sensation.